1. Field of the Invention
This invention relates to hearing protection earplugs and more particularly to self-fitting hearing protection earplugs with facile insertion mechanisms which also may possess an integral machine detectability characteristic and also may form the basis for an advanced high-noise environment communications system.
2. Description of Related Art
The need for adequate hearing protection in high noise environments has long been recognized among those concerned with health and safety issues, and much effort has gone into providing such protection. However, most scientists in this field would acknowledge that this effort has not been very successful. Protective devices have proliferated yet remained mediocre in performance. Workers in high noise environments who should use these devices often do not, or use them only under duress from their employers. Individuals that work in high noise environments rarely understand that the effects of high noise exposure are not limited to the moment but are cumulative as well. The lack of worker compliance with safety rules is exacerbated by the fact that currently available hearing protection devices are often uncomfortable, clumsy to use, and perform poorly. It is this second aspect of the problem that can be addressed through technology. Fortunately, as hearing protection devices become more comfortable and perform better, worker compliance with their use should also improve.
Hearing protection devices may be broadly divided into two categories: earcups and earplugs. Earcups are widely used in both industry and in the military. Recently, Poiesis Research, Inc., of Florida, working with the U. S. Navy Aerospace Medical Research Laboratory at Pensacola Fla., demonstrated an advanced earcup design with over 45 dB of attenuation at 100 Hz. This earcup was made possible by advances in materials science described in U.S. Pat. No. 5,400,296, and comes very close to being "perfect" in the sense that it is generally acknowledged that any more than 50 dB of attenuation would be masked by bone conduction and, therefore, superfluous. However, these test results were obtained in a laboratory situation. The earcup was placed against a polished stainless-steel flat-plate coupler that had been lightly coated with vacuum grease to insure a good seal. A real world user will not be likely to apply a coat of grease to insure a seal against his uneven head.
Even a "perfect" earcup will never mate well with the head of the average user, and objectionable noise will simply flank the earcup through leaks at the seal and reduce its effectiveness considerably. Furthermore, earcups of all types always require additional equipment to hold them in place, are heavy, apply pressure to the side of the users head that can cause discomfort, block evaporative cooling over the ears, interfere with other safety devices such as helmets, and may be dangerous in an accident if they are strongly constructed and act like a "cookie cutter."
Ultimately it is always the seal between the hearing protection device and the human using it that limits the overall effectiveness of the hearing protection approach being used. In the case of an earcup, an engineer can do very little about sealing against the user's head. It is the human, not the earcup, that limits this type of hearing protection. Earplugs, however, do not suffer from many of the problems associated with earcups and work within a human/machine interface that is much more amenable to technological improvement. Although the human external auditory meatus is irregular in shape, and contains short fine hairs, it also secretes an excellent sealant, the cerumen or "ear wax." The "ear wax" found on the internal wall of the human external auditory meatus insures that a soft compliant material pressed against it has an excellent chance of forming a very good seal. However, it must be remembered that the distal end of the ear canal contains an exquisitely sensitive pressure transducer that does not tolerate large pressure changes well. Any earplug that seals well may also act as a piston within the external auditory meatus when being inserted or withdrawn and cause discomfort, and sometimes extreme discomfort, as this piston compresses the air or causes a vacuum adjacent to the tympanum. A "perfect" earplug must seal well when in use but not seal or have a pressure release mechanism during insertion or withdrawal.
The "perfect" earplug should also attenuate close to 50 dB over the entire spectrum of audible sound. The materials described in U.S. Pat. No. 5,400,296 suggest that this is a realizable goal, albeit a very difficult one. The "perfect" earplug should be quick and facile to insert and remove. Facile insertion and removal will improve user compliance with hearing protection requirements in high noise environments. Three other aspects of the "perfect" earplug can be posed as design targets: a) The "perfect" earplug should be machine detectable so that if it should, for example, fall into a batch of cereal at a processing plant it can be easily retrieved. b) The design of the "perfect" earplug should be amenable to use as part of a two way communications device for high noise environments. And, c) The "perfect" earplug must be manufacturable. Experiments at Poiesis Research, Inc. have shown that when a microphone is placed in the external auditory meatus and shielded from external noise the same mechanisms that provide 50 dB of auditory shielding for the tympanum work for the microphone as well. The subject's vocal cords are acoustically coupled to the external auditory meatus and his voice is available for pickup by the microphone within this canal. Conversely, many microphone designs will also work as speakers, thus providing the basis for an excellent two-way communications system sealed into the external auditory meatus.
Prior art earplugs all fall short of the ideal posed above. For example, U.S. Pat. No. 2,246,737 shows an earplug made of resilient material with projecting annular fins which deform and engage the internal surfaces of the external auditory meatus. This earplug and its many related "annular fin" designs act as a piston within the external auditory meatus when being inserted and withdrawn, thus causing sometimes painful pressure differentials across the tympanum. U.S. Pat. No. 3,881,570 shows a self-fitting earplug that must be manually elongated by rolling between the fingers and stretching before insertion in the external auditory meatus where it will slowly contract lengthwise and expand radially to fill and seal the canal. The putty-like material within this earplug must be stiff and have a very slow recovery time to allow sufficient time for insertion after it has been elongated, thus making it difficult to elongate in the first place and making the time lag between insertion and sealing quite long and incomplete. Furthermore, this design also acts as a piston when being withdrawn. U.S. Pat. No. 4,089,332 shows a pneumatic shaping earplug with some promise, but this design is limited in the ratio of contracted to expanded states by the mechanism involved to a rather small difference. This design will also act as a piston when withdrawn, and possibly when inserted. U.S. Pat. No. 4,060,080 shows an earplug design that is much closer to the ideal proposed above except that it is necessarily liquid-filled and liquids are very poor attenuators. Furthermore, the design shown in U.S. Pat. No. 4,060,080 would be difficult to manufacture.